The present invention relates to a non-isolated buck-boost converter for converting an input DC voltage to a predetermined output voltage.
In a power supply circuit which receives a DC voltage (hereinafter referred to as an “input DC voltage”) from a DC power supply and outputs a DC voltage (hereinafter referred to as an “output DC voltage”) as a DC power supply voltage for various kinds of electronic circuits, a buck-boost converter is used when the input DC voltage varies with respect to the predetermined output DC voltage. Such a buck-boost converter is described in Japanese Laid-Open Publication No. 11-299229, for example.
Hereinafter, with reference to FIG. 12, the buck-boost converter described in the above-mentioned publication will be discussed as a conventional example. FIG. 12 illustrates the circuit configuration of the buck-boost converter of the conventional example. As shown in FIG. 12, the conventional buck-boost converter includes: a buck converter section 20, which includes a first switch 2, a first diode 3, and an inductor 4; a boost converter section 25, which shares the inductor 4 and includes a second switch 5, a second diode 6, and a capacitor 7; and a control circuit 30, which generates and outputs a first driving signal DR1 for opening and closing the first switch 2 and a second driving signal DR2 for opening and closing the second switch 5.
In the buck converter section 20, the input terminal of the first switch 2 is connected to an input DC power supply 1, such as a DC battery, which provides a voltage Vi, while the output terminal of the first switch 2 is connected in series with the first diode 3, and this series circuit is connected in parallel with the input DC power supply 1. The inductor 4 is connected to the junction of the first switch 2 and the first diode 3.
In the boost converter section 25, the second switch 5 and the second diode 6 are connected in series with each other, and this series circuit is connected in parallel with the capacitor 7. The inductor 4, which is shared with the buck converter section 20, is connected to the junction of the second switch 5 and the second diode 6. From the junction of the second diode 6 and the capacitor 7, an output DC voltage Vo is output to supply power to various kinds of electronic circuits (not shown).
The control circuit 30 includes an error amplifier circuit 31, an offset circuit 32, an oscillator circuit 33, a first comparator 34, and a second comparator 35. The error amplifier circuit 31 detects the output DC voltage Vo and produces a first control signal Ve obtained by amplifying an error between the output DC voltage Vo and a target value (a predetermined voltage). The potential of the first control signal Ve decreases when the output DC voltage Vo is higher than the target value, and increases when the output DC voltage Vo is lower than the target value. The offset circuit 32 subtracts an offset voltage Es from the first control signal Ve, thereby producing a second control signal Vy(=Ve−Es). The oscillator circuit 33 generates a triangular wave signal Vt that increases and decreases in a certain cycle, for example, at 500 kHz to 1 MHz. The amplitude of the triangular wave signal Vt is Et, and the amplitude (the potential difference) Et is set equal to or smaller than the offset voltage Es (Et≦Es). The first comparator 34 compares the triangular wave signal Vt with the first control signal Ve, and when the first control signal Ve is greater, the first comparator 34 produces the first driving signal DR1 that goes to the high (H) level. The second comparator 35, on the other hand, compares the triangular wave signal Vt with the second control signal Vy, and when the second control signal Vy is greater, the second comparator 35 produces the second driving signal DR2 that goes to the high (H) level.
FIG. 13 shows a timing chart (an operation waveform diagram) of the signals in the control circuit 30. As shown in FIG. 13, in the first half of the chart, the first control signal Ve intersects the triangular wave signal Vt intermittently, but the second control signal Vy does not intersect the triangular wave signal Vt. This causes the first driving signal DR1 produced by the first comparator 34 to become a pulsed signal and turn the first switch 2 on and off alternately. At this time, as the first control signal Ve is raised, the pulse width of the first driving signal DR1 increases. On the other hand, in this first half, the second driving signal DR2 produced by the second comparator 35 is at the low (L) level, such that the second switch 5 is in the off state.
In FIG. 12, when the first switch 2 alternates between the on and off operations and the second switch 5 is in the off state, the buck-boost converter operates as a buck converter. When the first switch 2 is in the on state, the differential voltage (Vi−Vo) between the input DC voltage Vi and the output DC voltage Vo is applied to the inductor 4 to cause current to pass from the input DC power supply 1 to the first switch 2, the inductor 4, and the second diode 6 in this order, whereby electric energy is stored in the inductor 4. On the other hand, when the first switch 2 is in the off state, the output DC voltage Vo is applied to the inductor 4 to cause current to pass through the first diode 3, the inductor 4, and the second diode 6 in this order, whereby the electric energy stored in the inductor 4 is discharged. Where the ratio (the duty ratio) of on-time (the pulse width of the first driving signal DR1) to one switching cycle of the first switch 2 (one cycle of the triangular wave signal Vt) is D1, the output DC voltage Vo is expressed by Vo=D1×Vi. As the first control signal Ve is raised, this duty ratio D1 increases. That is, when the input DC voltage Vi is higher than the output DC voltage Vo, the buck-boost converter operates as a buck converter and the duty ratio D1 is adjusted by the control circuit 30 so that the output DC voltage Vo becomes the target value.
As shown in FIG. 13, in the latter half of the chart, when the first control signal Ve increases and thus does not intersect the triangular wave signal Vt any more, the first driving signal DR1 output from the first comparator 34 is always at the high level, causing the first switch 2 to be kept in the on state. On the other hand, the second control signal Vy comes to intersect the triangular wave signal Vt intermittently, whereby the second driving signal DR2 output from the second comparator 35 becomes a pulsed signal to cause the second switch 5 to alternate between on and off. At this time, as the second control signal Vy is raised, the pulse width of the second driving signal DR2 increases.
In FIG. 12, when the first switch 2 is in the on state and the second switch 5 alternates between the on and off operations, the buck-boost converter operates as a boost converter. When the second switch 5 is in the on state, the input DC voltage Vi is applied to the inductor 4 to cause current to pass from the input DC power supply 1 to the first switch 2, the inductor 4, and the second switch 5 in this order, whereby electric energy is stored in the inductor 4. On the other hand, when the second switch 5 is in the off state, the differential voltage (Vi−Vo) between the input DC voltage Vi and the output DC voltage Vo is applied to the inductor 4 to cause current to pass from the input DC power supply 1 to the first switch 2, the inductor 4, and the second diode 6 in this order, whereby the electric energy stored in the inductor 4 is discharged. Where the ratio (the duty ratio) of on-time (the pulse width of the second driving signal DR2) to one switching cycle of the second switch 5 (one cycle of the triangular wave signal Vt) is D2, the output DC voltage Vo is expressed by Vo=Vi/(1−D2). As the second control signal Vy is raised, this duty ratio D2 increases. That is, when the input DC voltage Vi is lower than the output DC voltage Vo, the buck-boost converter operates as a boost converter and the duty ratio D2 is adjusted by the control circuit 30 so that the output DC voltage Vo becomes the target value.
The reason why the offset voltage Es is set equal to or greater than the amplitude Et of the triangular wave signal Vt is to prevent increase in switching loss caused by a mixture of the on and off operations of the first and second switches 2 and 5 within one switching cycle and to allow the buck-boost converter to operate in such a manner that the operation of the buck converter section 20 and the operation of the boost converter section 25 are separated.
As described above, in the buck-boost converter according to the conventional example, the control circuit 30 adjusts the duty ratio D1 or D2, whereby increase and decrease of the output DC voltage Vo with respect to the input DC voltage Vi can be controlled.
In the buck-boost converter according to the conventional example, it is when the amplitude Et of the triangular wave signal Vt is equal to the offset voltage Es that continuous transitions between the operation of the buck converter section 20 and the operation of the boost converter section 25 are made in accordance with variations in the input DC voltage Vi. However, in actual design, in order to operate the buck-boost converter in such a manner that the operation of the buck converter section 20 and the operation of the boost converter section 25 are separated, the offset voltage Es is set higher than the amplitude Et of the triangular wave signal Vt with consideration given to variation in the circuit constants. In this manner, when the offset voltage Es is set higher than the amplitude Et of the triangular wave signal Vt, neither the first control signal Ve nor the second control signal Vy intersects the triangular wave signal Vt, resulting in the occurrence of a through mode in which the first switch 2 is in the on state and the second switch 5 is in the off state. In the through mode, current passes across the input and output terminals of the buck-boost converter through the first switch 2, the inductor 4, and the second diode 6. In the real world, the operation of the buck-boost converter is not stable in the through mode, and slight variation in input/output conditions and environment causes switching between the through mode and the boost or buck operation. Such transient changes in the operation state are also affected by response speed of the control circuit 30 including the error amplifier circuit 31 and the like and are thus irregular, thereby causing a problem in that the output ripple of the output DC voltage Vo is increased.